(1) Field of the Invention
The present invention relates to a cassette size detecting mechanism, and more particularly, to a mechanism for detecting a size of a sheet cassette for accommodating cut sheets for copy or print used in copiers, printers and the like.
(2) Description of the Prior Art
In copiers, printers, etc., use of the sheet is made of cut sheets that are cut in advance in predetermined sizes such as, for example, B5, B4, A4, A3 of JIS. The cut sheets are in general accommodated in cassettes that can be attached to and detached from the body of a copier or the like. In order for the copier body to detect the size of sheets contained in a cassette, there is provided typically three cassette size detection blocks on a side of the cassette, and the shape of blocks or combination of recess and projection of the blocks is used to identify the size of sheets of, maximally, third power of 2, or eight kinds.
One example of such prior art cassettes is shown by a schematic side view in FIG. 1. In the figure, a cassette 1 is provided on its one side with a cassette size detection block set 2. Provided for the copier body is a cassette insertion guide frame 3, to which a printed-circuit board 4, tact switches 5, size detecting levers 6 and other members are equipped.
Typically, three spaces for blocks are provided for detection block set 2, and the combination of recessed and projected shapes of the blocks is adapted to correspond to a size of sheets held in a cassette.
On the other hand, cassette insertion guide frame 3 includes a printed-circuit board 4 disposed in parallel with the cassette side, as well as a guide frame. Further, there are provided on the printed-circuit board 4 a row of tact switches 5 in a position facing the above-mentioned detection block set 2. Caulked on the printed-circuit board 4 is actuators 6 each made of a leaf spring for turning on and off the tact switch 5 in link with insertion and withdrawal of the cassette (See FIG. 2). A state of leaf spring (actuator) 6 indicated by a broken line 6-1 shows one in which tact switch 5 is engaged in operation by a block of detection block set 2.
As the prior art cassette size detecting mechanism described above uses leaf spring actuators, if the insertion and withdrawal of the cassette is practiced roughly, the actuators might be deformed, or alternatively, if the cassette remains inserted in a prolonged period of time, the leaf springs for the actuators may be worn out to be deformed, causing failure in size detection.
The main cause of defective size detection as described above has been known to be attributed to the use of leaf springs that are subject to deform. Nevertheless, in order to solve the cause, there have been problems such that an expensive, complicated switch operating mechanism should be required.